is_hashable<T>::value is true iff std::hash<T>(T t) works or gcc is too
old

I see no harm in it atm.

-#include <dune/stuff/common/logging.hh>
+#include <dune/stuff/common/timedlogging.hh>
is encouraged, but not mandatory yet

This does not yet test whether the file has actually been created or
if it contains the correct stuff.

This commit should get rid of some warnings and small inconsistencies.
The LogStream class now derives from StorageProvider which holds the
buffer before the buffer is given to the ostream base class. FileLogStream
and EmptyLogStream now merely create the corresponding stream buffer
and forward it to LogStream. Thus overriding rdbuf does not seem necessary
any more.

This commit allows to call entityRange() for a either a grid view or a grid
part. In addition I renamed ViewRange -> EntityRange and
viewRange() -> entityRange() which is consistent with the existing
intersectionRange() and cornerRange()...

This can be used like:
```
DSC_LOG_INFO_TIMED("main") << "doing stuff..." << std::endl;
```